This invention generally relates to collet chucks, and is specifically concerned with a collet chuck having an anti-friction bearing between its nose ring and locknut that is loaded in a direction parallel with respect to the axis of rotation of the chuck.
Collet chucks having anti-friction bearings are well known in the prior art. Such chucks are typically comprised of a housing which is detachably connectable to the spindle of a turning tool which includes an opening for receiving a collet. The collet includes a series of parallel, longitudinally oriented cuts around its body for rendering it radially compressible around the shank of a tool, and terminates in a frustro-conically shaped outer edge which defines a follower surface. The outer edge of the collet projects out of the opening in the chuck housing, and is surrounded by a nose ring having a cam surface. The cam surface of the nose ring is complementary in shape to the follower surface of the collet, and wedgingly compresses the end of the collet radially inwardly when driven axially toward the housing of the chuck. The inner surface of the radially compressed collet then grips the shank of a rotatable cutting tool such as a drill or a milling cutter.
To drive the nose ring in the axial direction to radially compress the collet, a generally tubular locknut is provided around the chuck body. The inner diameter of one end of the locknut is threadedly engaged to the chuck body, and the opposite end of the locknut is rotatably connected to the nose ring by means of a bearing assembly. The use of a bearing assembly to reduce friction between the nose ring and the locknut is required in precision collet chucks designed to firmly grip the shanks of large cutting tools that are rotated at high speeds. Such collet chucks must use thick-walled collets that require large radial forces in order to overcome their resiliency and firmly engage them against the shank of a cutting tool. Without an anti-friction bearing between the nose ring and the locknut, the amount of torque required to screw the locknut to an axial position on the chuck housing for adequately compressing the collet would likely exceed the strength of the average machine tool operator. Even if it were possible for the operator to manually adequately turn the locknut, the large amount of torque that would have to be manually applied would, at the very least, greatly impede quick tool changes. To reduce the torque between the locknut and nose ring to a level that can be easily manually applied by the machine operator, prior art chuck collets have employed either needle bearings or ball bearings. Ball bearing arrangements are generally preferred by machine operators over needle bearing arrangements since the bottom portion of the locknuts used in conjunction with ball bearings do not obscure the outer edge of the collet as locknuts used with needle bearings do. However, despite this advantage, the applicants have noted a number of shortcomings in the designs of ball bearing chucks which could bear improvement.
For example, in virtually all of the prior art collet chucks that utilize ball bearings, the compressive forces transmitted between the locknut and the nose ring are applied at an angle which is non-parallel to the axis of rotation of the chuck housing. An example of such oblique loading across the ball bearings may be seen in U.S. Pat. No. 3,652,100. Because of the high compressive forces that the screw thread of the locknut is capable of generating in the axial direction, the angular orientation of the ball bearings in the nose ring of this particular chuck causes the bearings to generate a hoop stress to the bottom edge of the locknut adjacent to the bearings. Over time, these hoop stresses can deform the cylindrical profile of the locknut and interfere with its smooth functioning.
In another type of collet chuck disposed in U.S. Pat. No. 3,365,204, the ball bearings are disposed in opposing grooves present in the nose piece and locknut which are radially oriented such that the ball bearings serve to "key" the locknut and nose ring together. Unfortunately, in this design, the axially-oriented shear forces applied to the outer and inner halves of each of the ball bearings will tend to distort the shape of these grooves over time, which in turn will impair the smooth operation of the bearing. Even more importantly, this particular design requires that the ball bearings be inserted through a threaded bore located in the sidewall of the locknut to effect the assembly of the chuck. The bore is plugged by a set screw after all the ball bearings have been inserted into the opposing grooves. The necessity of installing the ball bearings through such a bore not only makes the assembly and disassembly of this collet chuck tedious and time consuming; it further creates a weight imbalance at a point on the circumference of the locknut which must be compensated for by some kind of balancing mechanism if the collet chuck is rotated at high speeds.
Still another shortcoming associated with such prior art collet chucks is the fact that in some of the grooves of the bearing assemblies, the ball bearings make only a point or a line contact with the surface of the grooves. Such point or line contact generates concentrated stresses in the ball recess in the nose ring and locknut which can ultimately distort the shape of the grooves, and again interfere with the smooth functioning of the bearing.
Additionally, in general, the cam surfaces of the nose ring do not always perfectly match the frustro-conical shape of the follower surfaces on the outer edge of the collet in many such prior art collet chucks. Such a mismatch in the shape of the cam and follower surfaces can in turn result in a nonuniform radial compression of the collet, which in turn can cause nonuniform gripping forces around the shank of the tool. Finally, there is no provision in such prior art collet chucks for preventing dust and debris from entering the bearing assembly between the nose ring and locknut, or for retaining lubricant therein, which again can impede smooth functioning and lead to life-shortening wear.
Clearly, there is a need for an improved collet chuck in which the transmission of reactive forces through the ball bearings does not create unwanted distortions or wear patterns in either the locknut, or the grooves that the bearings roll in. Ideally, such a collet chuck could be easily assembled without the need for a special ball-receiving bore in the wall of the locknut, which not only slows assembly, but further creates unwanted imbalances in the collet chuck. Such a collet chuck should also have some provision for insuring a uniform engagement between the cam surfaces of the nose ring and the follower surfaces around the outer edge of the collet, and some sort of means for both preventing the entry of dust and debris into the bearing assembly and retaining lubricant within the ball track space.